Gas turbine buckets have historically been either uncooled or cooled by compressor bleed air. For example, cooled aircraft and land-based gas turbine buckets have used compressor bleed air as the cooling medium to obtain service temperatures within the portion of the bucket exposed to the hot gas stream for acceptable creep and oxidation life. For firing temperatures up to about 2350.degree. F., the first-stage bucket has been cooled convectively by compressor bleed air in a serpentine passage arrangement with turbulence promoters within the passages to improve the rate of heat transfer with the flow exiting through trailing edge cooling holes. Similarly, the second-stage bucket for the same machine has been cooled by compressor bleed air flowing through machined radial holes with turbulence promoters to enhance heat transfer with the flow exiting through the tip. For firing temperatures between 2350.degree. F. and 2600.degree. F. in aircraft gas turbines, buckets have been cooled by compressor bleed air exiting through the leading edge and providing an insulating film of air about the buckets.
The foregoing-described cooling schemes, however, sacrifice compressor air flow and this puts practical limits on compressor pressure ratio, compressor efficiency, overall efficiency and overall plant heat rate. In land-based gas turbines used for electrical power generation, there are competing demands for the use of compressor bleed air, not only for cooling turbine components but also for dilution flow in the combustion system to lower oxides of nitrogen in the turbine exhaust. As the turbine firing temperatures increase, additional compressor bleed air is needed to cool the turbine components. Turbine buckets cooled with a boundary layer film of air place even greater demands for compressor bleed air. Turbine bucket cooling, in that case, may only be successfully employed at the expense of overall thermal efficiency. In aircraft gas turbines, those schemes, however, can be employed with reasonable success.
Further, in shrouded bucket designs, the shrouds have previously been uncooled. Coolant, such as compressor bleed air, exiting the airfoil tip is largely ineffective for cooling the shroud. With higher firing temperatures and higher bucket tip gas temperatures, the necessity for tip shroud cooling has arisen.